The present invention pertains generally to biomedical materials, and more particularly to preserved biological tissues, such as porcine bioprosthetic heart valves, which are implantable in a mammalian body.
The prior art has included numerous methods for preserving or fixing biological tissues, to enable such tissues to be subsequently implanted into mammalian bodies. Examples of the types of biological tissues which have heretofore been utilized for surgical implantation include cardiac valves, vascular tissue, skin, dura mater, pericardium, ligaments and tendons.
The term xe2x80x9cgraftingxe2x80x9d as used herein is defined as the implanting or transplanting of any living tissue or organ (See Dorlands Illustrated Medical Dictionary, 27th Edition, W.B. Saunders Co. 1988). Biological tissues which are grafted into the body of a mammal may be xenogeneic (i.e., a xenograft) or allogeneic (i.e., an allograft).
The term xe2x80x9cbioprosthesisxe2x80x9d defines many types of biological tissues chemically pretreated before implantation (Carpentierxe2x80x94See Ionescu (editor), Biological Tissue in Heart Valve Replacement, Butterworths, 1972). As opposed to a graft, the fate of a bioprosthesis is based upon the stability of the chemically treated biological material and not upon cell viability or host cell ingrowth. Chemical pretreatment includes the xe2x80x9cfixingxe2x80x9d or tanning of the biological tissue. Such fixing or tanning of the tissue is accomplished by exposing the tissue to one or more chemical compounds capable of cross-linking molecules within the tissue.
Various chemical compounds have been utilized to fix or cross-link biological tissues including formaldehyde, glutaraldehyde, dialdehyde starch, hexamethylene diisocyanate and certain polyepoxy compounds.
In particular, glutaraldehyde has proven to be relatively physiologically inert and suitable for fixing various biological tissues for subsequent surgical implantation (Carpentier, A., J. Thorac. Cardiovasc. Surg. 58:467-68 (1969)). In particular, examples of the types of biological tissues which have heretofore been subjected to glutaraldehyde fixation include porcine bioprosthetic heart valves and bovine pericardial tissues.
Clinical experience has revealed that glutaraldehyde-fixed bioprosthetic tissues may tend to become calcified. Such calcification of glutaraldehyde-fixed bioprosthetic tissues has been reported to occur most predominantly in pediatric patients see, Carpentier et al. and xe2x80x9cContinuing Improvements in Valvular Bioprostheses, J. Thorac Cardiovasc. Surg. 83:27-42, 1982. Such calcification is undesirable in that it may result in deterioration of the mechanical properties of the tissue and/or tissue failure. In view of this, surgeons have opted to implant mechanical cardio-vascular valves into pediatric patients, rather than to utilize glutaraldehyde-preserved porcine valves. However, pediatric patients who receive mechanical valve implants require long term treatment with anticoagulant medications and such anticoagulation is associated with increased risk of hemorrhage.
The mechanism by which calcification occurs in glutaraldehyde-fixed bioprosthetic tissue has not been fully elucidated. However, factors which have been thought to influence the rate of calcification include:
a) patient""s age
b) existing metabolic disorders (i.e., hypercalcemia, diabetes, kidney failure . . . )
c) dietary factors
d) race
e) infection
f) parenteral calcium administration
g) dehydration
h) distortion/mechanical factors
i) inadequate coagulation therapy during initial period following surgical implantation; and
j) host tissue chemistry
Various efforts have been undertaken to find ways of mitigating calcification of glutaraldehyde fixed bioprosthetic tissue. Included among these calcification mitigation techniques are the methods described in U.S. Pat. No. 4,885,005 (Nashef et al.) SURFACTANT TREATMENT OF IMPLANTABLE BIOLOGICAL TISSUE TO INHIBIT CALCIFICATION; U.S. Pat. No. 4,648,881 (Carpentier et al.) IMPLANTABLE BIOLOGICAL TISSUE AND PROCESS FOR PREPARATION THEREOF; U.S. Pat. No. 4,976,733 (Girardot) PREVENTION OF PROSTHESIS CALCIFICATION; U.S. Pat. No. 4,120,649 (Schechter) TRANSPLANTS; U.S. Pat. No. 5,002,566 (Carpentier) CALCIFICATION MITIGATION OF BIOPROSTHETIC IMPLANTS; EP 103947A2 (Pollock et al.) METHOD FOR INHIBITING MINERALIZATION OF NATURAL TISSUE DURING IMPLANTATION and WO84/01879 (Nashef et al.) SURFACTANT TREATMENT OF IMPLANTABLE BIOLOGICAL TISSUE TO INHIBIT CALCIFICATION.
There remains a need for the development of new methods for inhibiting or mitigating calcification of chemically-fixed biological tissue.
It is postulated that tissue calcification may be minimized by accelerating the polymerization of glutaraldehyde solution coming into contact with the tissue prior to implantation.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a method for treating glutaraldehyde fixed biological tissue or biological tissue fixed with other chemicals so as to inhibit later calcification of the tissue following implantation of the tissue into a mammalian body. The method comprises placing the biological tissue in contact with glutaraldehyde or another chemical fixative and then heating the glutaraldehyde or other fixative and/or causing the glutaraldehyde or other fixative to be polymerized by thermal, chemical or mechanical means.
In the preferred embodiment of the present invention the biological tissue is disposed within a container containing a 0.625% solution of glutaraldehyde comprising approximately 26 ml/l glutaraldehyde (25%); approximately 4.863 g/l HEPES buffer; approximately 2.65 g/l MgCl2 .6H2O; and approximately 4.71 g/l NaCl. The balance of the solution comprises double filtered H2O. Sufficient NaOH is added to adjust the pH to approximately 7.4.
The glutaraldehyde solution is heated to between approximately 35-55xc2x0 C. for approximately 4-22 weeks.
The biological tissue may be heat treated any time prior to implantation thereof within a mammalian body. For example, the tissue may be treated before fixing thereof. Treatment before fixing of the biological tissue merely involves heating it in a saline solution (9 g/l NaCl) or any other physiologic solution at 25-80xc2x0 C. for a few seconds to 22 weeks. Alternatively, the tissue may be treated during fixing thereof, while the tissue is disposed within a glutaraldehyde solution. Treatment during fixing of the biological tissue merely involves heating of the glutaraldehyde solution to approximately 25-80xc2x0 C. for approximately a few seconds to several months. The preferred range is 35 to 55xc2x0 C. for 4-22 weeks.
Alternatively, the biological tissue is treated after fixing thereof, and before storage thereof. Such treatment is preferably accomplished while the biological tissue remains within the glutaraldehyde solution utilized during the fixing process and/or is disposed with a glutaraldehyde solution within which the biological tissue is to be stored and again merely comprises heating of the glutaraldehyde solution to approximately 25-80xc2x0 C. for approximately 4-22 weeks.
Alternatively, the biological tissue is treated after storage thereof, typically a short time prior to implantation within a mammalian body. The biological tissue is preferably heated within the glutaraldehyde solution within which it has been stored by merely heating the glutaraldehyde solution to approximately 35-55xc2x0 C. for approximately 4-22 weeks.
Alternatively, the biological tissue is treated during an antimineralization process by adding glutaraldehyde to the antimineralization solution and heating, preferably to approximately 35-55xc2x0 C. for approximately 4-22 weeks. As those skilled in the art will recognize, various antimineralization processes are utilized to inhibit mineralization of the biological tissue by calcium and various other minerals.
Heating of the glutaraldehyde appears to effect polymerization thereof. It is believed that the inhibition of calcification is due to the polymerization of glutaraldehyde contained within the biological tissue. As such, those skilled in the art will appreciate that various other cross linking agents and other processes, such as light, radiation, or chemicals, which effect polymerization of glutaraldehyde may likewise be utilized in the method for treating biological tissue of the present invention. Thus, various chemical fixatives other than glutaraldehyde and other methods for effecting polymerization of these chemicals and/or glutaraldehyde may be used alone, or in combination with heat, so as to effect polymerization of the chemicals contained within the biological tissue, thereby inhibiting later calcification of the tissue after implantation thereof in a mammalian body.
The method of inhibiting calcification of fixed biological tissue may be utilized with various different types of biological tissue such as cardiac valves, vascular tissue, skin, dura mater, pericardium, fascias, ligaments, and tendons. Those skilled in the art will realize that this list is not comprehensive in that various other types of biological tissue may also benefit from treatment thereof according to the method of the present invention.